Magnesium-lithium-thorium alloys



This invention relates to magnesium base alloys and more particularly to magnesium base alloys containing lithium and thorium. Further, it concerns the beta phase of magnesium-lithium alloys employing thorium.

Recent developments, particularly in the aircraft industry, wherein stressed parts are subjected to elevated temperatures, have made it desirable to provide metals of light weight which have good stability and hardness at these elevated temperatures. Although the magnesium base alloys are among the lightest of the structural alloys, they have not met with wide acceptance in applications where stressed parts are exposed to elevated temperatures. Previous work has been done with magnesium-lithium alloys in an efrort to increase hardness of the alloy at elevated temperatures without loss of the desirable properties such as lightness and ductibility. These desired results are accomplished in this invention, wherein the magnesium-lithium alloy is combined with thorium to form an alloy with good stability and hardness at elevated temperatures.

Therefore, it is an object of this invention to provide an improved magnesium base alloy.

it is a further object of this invention to provide a magnesium base alloy with improved hardness at elevated temperatures.

It is yet another object of the instant invention to provide a beta phase magnesium-lithium alloy with good stability at elevated temperatures.

These and other objects of this invention will become evident upon reading the more detailed description hereinbelow.

In general the objects of this invention are accomplished by alloying magnesium and lithium in the ratio by weight of 7.5 magnesium to 1 lithium, and adding amounts of thorium not to exceed 5 percent of the total weight of the alloy. The ratio of 7.5 to 1 is necessary to insure the formation of the beta phase of the alloy wherein the lithium content cannot exceed 12 percent of the total Weight of the alloy.

The order of addition of the metals to the magnesium is immaterial. The magnesium and lithium may be melted together and the thorium added or the thorium may be added to the molten magnesium prior to the addition of the lithium.

In accordance with the invention to 12 percent by weight lithium and 2 to 4 percent thorium are alloyed with magnesium to make a 100 percent alloy. The magnesium and lithium are melted in an open top crucible with a flux protective cover. Any suitable flux known in the prior art can be used in this operation. The crucible is equipped with a stirring rod having a 1 inch fiat disk fastened to the end. The thorium is puddled into the solution using this stirring rod to insure complete stirring of the melt and better control over the addition of the thorium. After alloying has been effected, the melt is allowed to remain undisturbed for a time to permit separation of the flux as well as unalloyed metal, if any, thereby obtaining a clean melt of the alloy. This period is not required if the argon mantle is used. The settled alloy then is transferred into a suitable mold to solidify.

Small amounts of a metal selected from the group consisting of aluminum, zinc, and zirconium may be employed in the beta phase magnesium-lithium-thorium al- Patented June 15, 1%65 ice Example 1 A mixture of 86.5 grams of magnesium and 11.5 grams of lithium are melted in an open top crucible with a fiu) protective cover. A suitable flux is one composed of 53 parts of KCl, 28 parts of CaCl 12.5 parts of BaCl ant 2.5 parts of CaF Two grams of thorium are added tc the alloy using a stirring rod with a one inch diameter fia disk fastened to the end. This allows for complete stirring of the melt and better control over the addition of tho rium. After alloying has been effected, the melt is allowed to remain undisturbed for a time to permit separation 01 flux as well as unalloyed metal, if any, thereby rendering a clean melt of the alloy. The settled alloy is then transferred to a suitable mold, such as a sand or a metal mold to solidify. The ingots (l /2" x 1% x 3") are subsequently hot forged at a temperature of 450 F. to a A inch thick plate.

Example 2 A mixture of 84.7 grams of magnesium, 11.3 grams of lithium, and 4 grams of thorium are alloyed in the same manner as Example 1.

Example 3 A mixture of 85.6 grams of magnesium, 11.4 grams oi lithium, 2 grams of thorium, and 1 gram of zirconium are alloyed in the same manner as Example 1. The zirconiurr is added in the same manner as the thorium. The alloy is forged as in Example 1 but at a temperature of 500 F Example 4 A mixture of 82.1 grams of magnesium, 10.9 grams o1 lithium, 2 grams of thorium, and 5 grams of zinc are alloyed as in Example 3. The alloy is forged at a temperature of 550 F.

Example 5 A mixture of 81.2 grams of magnesium, 10.8 grams oi lithium, 4 grams of thorium, and 4 grams of aluminum are alloyed as in Example 3. The alloy is forged at 550 F The following table illustrates the hardness of each 01 the alloys in the above examples. An alloy of 12 percent lithium and 88 percent magnesium is used as a reference.

Diamond Pyramid Forg- Hardness kgJmm.

Nominal Alloy Composition, mg

Wt. Percent Temp F F F 178 450 33. 2 -1 4. 2 0. i 181 450 39. (l 14. 2 5. 1 1. 1 200 450 30. 9 17. 5 8. 4 4.. 500 33. 4 13.8 7. 0 4. l 194 550 25. 7 28. 2 20. 8 11. l 193 550 25. 2 21. 8 14. 2 7. t

3 4 lging from 2 to 4 percent by weight, and the remainder 5. The alloy of claim 3 wherein said metal is zinc. vgnesium. 6. The alloy of claim 3 wherein said metal is aluminum. a A g g f h t g alloy t mi: g 7 gT References Cited by the Examiner ra 10 y weig 0 magnesium 0 mm o l a thorium content varying from about '2 to 4 percent 5 FOREIGN PATENTS the tot l weight fth ll 681,264 10/ 52 Great Britain 75-168 5. A magnesium base alloyconsisting of .a fixed ratio 683,812 12/52 Great Bfltalll 75168 weight of magnesium to lithium, of v7.5/:1,ta thorium OTHER REFERENCES ltent varying from about .2 to 4 percent of the total 10 lght of the l y and add1 nna1'neta1 selected ,from aration, Fabrication, and General Characteristics, Metal group consisting of zlrconium, z1nc, and aluminum Transact'ons, F bru 1949, 149-168. Tying from about 1 to 5 percent of the total weight of I 6 my a11oy CARL D. QUARFORTH, Prlmary Exammer.

1. The alloy of claim 3 wherein said metal is zirconium. REUBEN EPSTEIN, Examiner.

Jackson -et al.: Magnesium-Lithium Base Alloys-Prep- 

1. A MAGNESIUM-LITHIUM-THORIUM GROUP METAL ALLOY HAVING BETTER STABILITY AND HARDNESS AT ELEVATED TEMPERATURES THAN MAGNESIUM-LITHIUM ALLOY OF SIMILAR COMPOISITON, SAID ALLOY CONSISTING ESSENTIALLY OF LITHIUM IN AMOUNTS RANGING FROM 10 TO 12 PERCENT BY WEIGHT, THORIUM IN AMOUNTS RANGING FROM 2 TO 4 PERCENT BY WEIGHT, AND THE REMAINDER MAGNESIUM. 